In an information reading optical head device for an optical disk such as a CD or a DVD, for example, a polarizing diffraction grating 500 shown in FIG. 15 is used as a polarizing beam splitter. The polarizing diffraction grating comprises a diffraction grating 1 made of a birefringent material layer having an ordinary refractive index no and an extraordinary refractive index ne (no≠ne) formed on one side of a glass substrate which is a transparent substrate 4, and the diffraction grating 1 has a periodical structure of concavo-convex shape with a step height d in cross section.
The concavo-convex portion of the periodical structure is filled with a homogeneous refractive index transparent material 3 having a refractive index ns substantially equal to the ordinary refractive index no so that the concavo-convex portion is leveled, and a glass substrate as a transparent substrate 5 is overlaid on the homogeneous refractive index transparent material 3 to form the polarizing diffraction grating 500. Here, |ne−ns|×d is made to be a half of the wavelength λ of incident light, whereby a polarizing diffraction grating is obtained, in which an ordinary polarized incident light (polarized in the direction providing ordinary refractive index) is straightly transmitted without being diffracted, and an extraordinary polarized incident light (polarized in the direction providing extraordinary refractive index) is diffracted and is not straightly transmitted.
There has been a problem that a sufficient extinction ratio can not be obtained when such a polarizing diffraction grating is used as an isolator for optical communication using a wavelength band of 1400 to 1700 nm. Namely, provided that the intensity of a first linearly polarized light (for example, ordinary polarized light) straightly transmitted is I1 and the intensity of a second linearly polarized light (extraordinary polarized light) straightly transmitted and polarized in a direction perpendicular to the polarization direction of the first linearly polarized light is I2, a ratio I2/I1 (hereinafter referred to as extinction ratio) of light of given single wavelength λ0 becomes at most −20 dB. However, since the transmittance of the straightly transmitted light of extraordinary polarized light is expressed by cos2 (0.5×π×λ0/λ), component of the incident light straightly transmitted without being diffracted is increased and the extinction ratio is deteriorated as the wavelength λ is away from λ0.
Further, in order to achieve a higher extinction ratio for a given single wavelength, it is necessary to accurately form the step height d of the periodical structure having a concavo-convex shape, and it has been difficult to obtain a polarizing diffraction grating having a high extinction ratio with good reproducibility.
Further, an example of a conventional optical attenuator employing liquid crystal is shown in FIG. 16. The optical attenuator is constituted by a liquid crystal cell 210 comprising transparent substrates 15 and 16 on which transparent electrodes 13 and 14 are formed, and a liquid crystal layer 11 of nematic liquid crystal in which the alignment direction of liquid crystal molecules is in parallel with the substrate surfaces and at an angle of 45° to the X-axis, sandwiched between the transparent substrates 15 and 16 and sealed inside a sealing member 18 provided at the peripheries of the substrates; and a polarizer 9 disposed at the light output side of the liquid crystal cell, which transmits only linearly polarized light polarized in X-axis direction.
Here, an AC power source 19 is connected to the transparent electrodes 13 and 14 to supply rectangular waves, and the thickness of the liquid crystal layer 11 is determined so that the retardation value of the liquid crystal cell 210 becomes about λ/2 for a linearly polarized light having a wavelength λ and polarized in the direction of Y-axis, when the voltage is not applied by the power source. Here, the purpose of setting the retardation value of the liquid crystal layer 11 to be about λ/2, is to minimize the insertion loss of the optical attenuator when the voltage is not applied, and to make the optical attenuator function as a λ/2 plate.
In this optical attenuator, the linearly polarized light polarized in the direction of Y-axis transmitted through the liquid crystal layer when the voltage is not applied between the transparent electrodes, becomes a linearly polarized light polarized in the direction of X-axis and is transmitted through the polarizer. When the voltage is applied, the alignment direction of liquid crystal molecules are tilted in the direction of the thickness of the liquid crystal layer, namely tilted perpendicularly to the substrates, as the applied voltage is increased. Accordingly, the retardation value of the liquid crystal cell is decreased and the light transmitted through the liquid crystal cell 210 becomes an elliptically polarized light. As a result, since the intensity of the transmitted light through the polarizer is simply decreased by the increase of the applied voltage, the optical attenuator is of a voltage variable type.
In a case of an optical attenuator employing a liquid crystal element, for optical communications using incident light having a wavelength of, for example, 1300 to 1600 nm, it is necessary to make the liquid crystal layer thicker than that of an optical attenuator for a visible wavelength region in order to make the retardation value of the liquid crystal cell to be λ/2. As a result, there has been a problem that a polarized light component transmitted through the polarizer remains, and therefore, an optical attenuator having a high extinction ratio can not be obtained, since even if an AC voltage having a voltage amplitude of at least 10 V is applied, the alignment direction of the liquid crystal molecules is not sufficiently oriented in the direction of the thickness of the liquid crystal layer and the retardation value of the liquid crystal cell does not become zero.
Further, FIG. 17 shows an example of conventional liquid crystal element for rotating the polarization direction of incident light as a linearly polarized light in accordance with the magnitude of an applied voltage.
The liquid crystal element is constituted by a liquid crystal cell 210 comprising transparent substrates 15 and 16 on which transparent electrodes 13 and 14 are formed, a liquid crystal layer 11 of nematic liquid crystal in which the alignment direction of liquid crystal molecules is in parallel with the substrate surfaces and in the direction at 45° to X-axis, the liquid crystal layer being sandwiched between the substrates and sealed by a sealing member 18; and a phase plate 10 made of a birefringent crystal having a fast axis or a slow axis in the direction of X-axis disposed at the light output side of the liquid crystal cell 210. Here, an AC power source 19 for generating rectangular waves is connected to the transparent electrodes 13 and 14, the thickness of the liquid crystal layer 11 is determined so that the retardation value R of the liquid crystal cell 210 for the linearly polarized incident light having a wavelength λ and polarized in the direction of X-axis when the voltage is not applied, is substantially λ/2, and the retardation value of the phase plate 10 is λ/4.
In this liquid crystal element, when the voltage is not applied between the transparent electrodes 13 and 14, the light transmitted through the liquid crystal layer becomes a linearly polarized light polarized in the direction of Y-axis, and is transmitted through the phase plate maintaining the state of linear polarization in the direction of Y-axis since the polarization direction coincides with either the slow axis or the fast axis of the phase plate 10. As the applied voltage is increased, the alignment direction of the liquid crystal molecules is tilted in the direction of the thickness of the liquid crystal layer. Accordingly, the retardation value R of the liquid crystal layer is decreased and the light transmitted through the liquid crystal cell 210 becomes an elliptically polarized light. Here, the polarization direction is rotated in accordance with the retardation value R of the liquid crystal layer maintaining the state of linear polarization when the light is transmitted through the phase plate 10.
The phase plate 10 to be employed for such a liquid crystal element is generally a birefringent crystal such as a quartz processed to have a thickness of at least 0.3 mm. However, in the case of a birefringent crystal, there has been a problem that the retardation value depends strongly on the incident angle as an angle between the propagation direction of the incident light and the normal line of the phase plate, which causes variation of the retardation value in the device plane for converging rays or diverging rays, and accordingly, polarization of the output light is not consistent. Further, since the retardation value has a dependency on wavelength, there has been a problem that when the incident light has a bandwidth in the wavelength, the linearity of the linearly polarized incident light is deteriorated when it is output from the element.
Considering the above-mentioned circumstances, it is an object of the present invention to provide a multi-layer diffraction type polarizer and a liquid crystal element capable of realizing a stable and high extinction ratio.
Further, considering the above-mentioned circumstances, it is another object of the present invention to provide a liquid crystal element for rotating the polarization direction of a linearly polarized light incident on the device and outputting the light maintaining the high linearity.